Capillary electrophoresis (CE) is a separation technique based on the differential migration of charged particles in an electric field. A thin capillary (20-100 .mu.m internal diameter) is filled with an electrolyte providing a medium in which analytes can migrate through. The sample is introduced at one end of the CE unit. An electric field typically of 100-600 V cm.sup.-1 is applied across the capillary facilitating analyte species migration according to their electrophoretic mobility (u) passing a detector as they migrate (usually UV or fluorescence) at or near the end of the capillary.
This separation technique as well as others, such as packed capillary liquid chromatography, capillary electrochromatography and super critical chromatography, require spectroscopic measurements to be made on extremely small volumes of flowing liquid samples. The typical application has a sample flowing through a fused silica capillary tube where inside diameters range from 15 to 150 micrometers and the outside diameters range from 150 to 300 micrometers. Various techniques presently are used for directing light from a suitable source into and/or through such a small volume sample cell, as well as taking the light emanating from the inside of the cell and directing it toward a light detecting or analyzing instrument to effect optical analysis or detection of samples contained in the cell. Alignment of the optical system to efficiently direct the light from the source to the capillary cell, particularly to the bore and sample therein, and/or to direct the radiation emanating from the cell to a detector, presents problems.
The underlying problem is generally related to selecting components and precisely aligning them for the purpose of directing light from a light source such as a laser to, or through, a volume of interest which has a small cross-sectional area perpendicular to the optical access. Similar problems are associated with collecting the light that emanates from a volume of interest and directing it to a photodetector or analyzer.
An implementation described in U.S. Pat. No. 5,037,199 to Hlousek utilizes an optical scheme which attempts to solve these focusing and alignment problems. In Hlousek's disclosure, a laser beam is focused into the lumen of a separations capillary or cell using a ball lens. The ball lens and the capillary/cell are mounted together as a unit. A lens focuses light from a source onto the ball lens. The size and shape of the light may be controlled and/or selected by placement of one or more suitably shaped apertures on axis with the source and the capillary/cell. The sphere or ball lens concentrates light by acting as a very short focal length lens to convert slowly converging light from the laser beam source to a rapidly converging cone of light that will image the source into or through the volume of interest in the cell.
A ball lens used this way suffers severe aberrations, in particular, spherical aberration and coma, making the laser focus larger than desirable. Refraction of light rays at the cylindrical outside surface of the capillary causes astigmatism and further enlargement of the focal spot. In addition, light is lost due to surface reflections at the ball-lens-to-air and air-to-capillary interfaces. Consequently, efficiency of the fluorescence excitation of the sample suffers. Similarly, the ability to efficiently collect the beam is negatively impacted.